Silicon nitride (Si.sub.3 N.sub.4) sintered bodies have excellent corrosion and heat resistance and high strength and, hence, have been used as various high-temperature materials. Further, owing to their advantages of good insulating properties and high chemical stability, silicon nitride sintered bodies have been used also as insulating substrate materials for electronic circuits such as ICs and the like.
In particular, with the recent trend in semiconductors toward higher degrees of integration and larger capacities, various attempts have been made to obtain insulating substrate materials having high thermal conductivity and, as a result, BeO sintered bodies with high thermal conductivity have been developed, followed by AlN sintered bodies and SiC sintered bodies both having good thermal conductivity.
However, the AlN sintered bodies have a drawback that they form a hydroxide in high-temperature steam to have impaired insulating properties, while the SiC sintered bodies have the inherent problem of high dielectric constants. Because of these drawbacks, both the AlN and SiC sintered bodies are at present used in only limited applications.
On the other hand, Si.sub.3 N.sub.4 sintered bodies have high strength, but it has been difficult to obtain Si.sub.3 N.sub.4 sintered bodies with high thermal conductivity. The reason for this is as follows. In general, a high degree of densification is difficult for Si.sub.3 N.sub.4. Further, Si.sub.3 N.sub.4 sintered bodies have conventionally been produced by liquid-phase sintering employing a sintering aid because high-temperature sintering poses the problem, for example, of the decomposition or sublimation of Si.sub.3 N.sub.4. In the liquid-phase sintering, Al.sub.2 O.sub.3 is used as the sintering aid to produce Sialon compounds in order to attain improved high-temperature strength and good resistance to oxidation at high temperatures. Thus, most studies have not been directed toward the development of higher-purity Si.sub.3 N.sub.4 sintered bodies. It has also been difficult to synthesize Si.sub.3 N.sub.4 powders having high purities.
Therefore, in the conventional Si.sub.3 N.sub.4 sintered bodies, heteroatoms such as Al and O which were contained in the sintering aids or Si.sub.3 N.sub.4 powders have come into the sintered bodies and such heteroatoms remain as intergranular layers or are present in the Si.sub.3 N.sub.4 lattices to form solid solutions. Accordingly, the thus-obtained Si.sub.3 N.sub.4 sintered bodies cannot fully exhibit the properties originally possessed by Si.sub.3 N.sub.4 and their thermal conductivities reported so far are normally as low as about 15 W/mK.
Because of such low thermal conductivities, Si.sub.3 N.sub.4 sintered bodies have not yet been put to practical use as electronic circuit substrates, although this application has been proposed in JP-A-62-30663. (The term "JP-A" as used herein means an "unexamined Japanese patent application".)